Breeding of fruity vegetables like peppers aims at the production of commercial varieties optimally adapted to a professional production environment in order to produce marketable products. Many characteristics need to be taken into account during selection which relate to both input and output traits. One of the very important traits in this respect relates to fruit set, in particular to fruit set under unfavourable environmental conditions such as high or low temperatures and drought. Such conditions can be detrimental for normal pollination and thereby fertilisation, which leads to poor fruit set and as a consequence yield loss. When fruit formation in the absence of fertilisation or parthenocarpy can be harnessed as a trait, it is an important characteristic that can significantly contribute to an economically more efficient production of pepper fruits for several reasons.
In addition to contributing to harvest security, parthenocarpy is also important for fruit quality. Blossom-end rot (BER) is a physiological disorder caused by a local calcium deficiency during the initial stage of fruit development. BER is of economic importance since it causes high yield losses in pepper. The occurrence of BER is strongly correlated with a high initial growth rate, but also with the number of seeds in a fruit. Parthenocarpic peppers are reported to be less susceptible to BER since they do not contain any seeds (Heuvelink and Körner; Annals of Botany 88: 69-74, 2001).
Furthermore, industrial or domestic applications which require seed removal from the fruits can benefit strongly from parthenocarpy. In the food processing industry seeds are usually removed by washing the seeds away with water. Since this step is redundant for the processing of parthenocarpic peppers there are less processing costs involved. Also for domestic applications parthenocarpic fruits have advantages over seeded fruits, because it is easier for consumers to process the fruits due to the absence of seeds.
The formation of fruits in the absence of fertilisation gives rise to the possibility to combine this character with a genetic source of male sterility, in particular GMS (Genetic Male Sterility). This form of male sterility causes anthers not to develop, therefore no pollen are produced. This has several advantages for the grower. First of all, the undeveloped anthers will decrease the chances for personnel to develop allergic reactions to pollen. Secondly, a cost reduction can be made because bumblebees or other pollinators normally used to optimize fruit set are not necessary anymore, since fruits will set in the absence of fertilisation.
Another high valued trait of parthenocarpy is that it stabilizes the production flow of pepper fruits. Normally, the production of peppers is characterized by periods with high production of fruits alternated with periods with low production. This phenomenon in the pepper production chain is called flushing. This cyclic behaviour leads to periods with high supply and low prices and periods with a low supply and high prices. Avoiding this irregular yield pattern in the pepper production process and providing a more stable production of peppers is of great economic importance. The fluctuations in pepper production are mainly caused by fluctuations in fruit set. The presence of several developing fruits set at approximately the same time inhibit the fruit set and growth of new fruits because of competition for resources and dominance due to the production of plant growth regulators.
Furthermore, it is known that an increase in the number of seeds in a pepper also increases the inhibitory effect of a fruit on set and growth of later developing fruits. Hence, the fluctuations in pepper production will be less by growing parthenocarpic fruits, which do not contain seeds (Heuvelink and Korner; Annals of Botany 88: 69-74, 2001).
Fruit set normally depends on fertilisation. Fertilisation is the process in which both the egg cell and central cell contained within the ovule are fused with a sperm cell delivered by the pollen tube. This so-called double fertilisation is the step which triggers a cascade of events leading to the formation of the embryo and endosperm and finally to a mature seed. The developing seeds and surrounding tissues generate a signal which stimulates the outgrowth of the ovary and its development into a fleshy fruit. Apparently fertilisation lifts a certain developmental barrier which prevents fruit formation. This mechanism assures the fruit formation to be dependent on the formation of seeds which makes sense given the biological role of fruits in seed dispersal.
However, the knowledge of the physiological and molecular events which play a role in the initial steps of fruit formation is fragmentary. The involvement of the plant hormones auxin and gibberellin has been extensively documented although their precise role remains elusive. The application of either auxin or gibberellin to the unfertilised ovule leads in many plant species including pepper to fruit formation. In fact, these hormones are applied in practice to improve fruit set when greenhouse conditions are suboptimal. Although the application of auxin and gibberellin has some practical value it increases cost and it may lead to irregularities in fruit shape.
In addition to these exogenous effects, it is assumed that the hormones auxin and gibberellin also play a role during fertilisation dependent fruit formation although it is not clear which tissues are the actual source of these hormones. The hierarchy of these hormones as well as the downstream regulatory network is still largely unknown. Other hormones such as cytokinins, abscisic acid, ethylene and brassinosteroid also seem to play a role in fruit formation.
Most pepper genotypes exhibit a certain level of parthenocarpy. However, these parthenocarpic fruits show negative pleiotropic effects, such as a reduced fruit size, irregular and deformed shape of the fruit, and carpelloid growth inside the fruit (Tiwari et al. BMC Plant Biology 11:143, 2011). Carpelloid growth is the outgrowth of a pseudo-embryo inside a pepper fruit, which is a highly undesired effect for both grower and consumer. Parthenocarpic fruits showing negative pleiotropic effects are called knots and are not to be considered as real parthenocarpic fruits.
Given the significant advantages of parthenocarpy for the production of pepper fruits there is a strong need for a genetic source of parthenocarpy in pepper which allows the formation of fruits in absence of fertilisation, better known as parthenocarpic fruit formation, which is preferably not linked to negative pleiotropic effects.
Citation or identification of any document in this application is not an admission that such document is available as prior art to the present invention.